Code Division Multiple Access (CDMA) communication systems are well known. In a CDMA communication system, communication between two communication units (e.g., a central communication site and a mobile communication unit) is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code. Due to the spreading, transmitted signals are in the same frequency band of the communication channel and are separated only by unique user spreading codes. These unique user spreading codes preferably are orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. Consequently, when the user spreading codes are orthogonal to one another, the received signal can be correlated with a particular user spreading code such that only the desired user signal (related to the particular spreading code) is despread.
It will be appreciated by those skilled in the art that several different spreading codes exist which can be used to separate data signals from one another in a CDMA communication system. These spreading codes include, but are not limited to, pseudo noise (PN) codes and Walsh codes. A Walsh code corresponds to a single row or column of the Hadamard matrix. For example, in a 64 channel CDMA spread spectrum system, particular mutually orthogonal Walsh codes can be selected from the set of 64 Walsh codes within a 64 by 64 Hadamard matrix. Also, a particular data signal can be separated from the other data signals by using a particular Walsh code to spread the particular data signal.
It will be further appreciated by those skilled in the art that spreading codes can be used to channel code data signals. The data signals are channel coded to improve performance of the communication system, and particularly radiotelephone communication systems, by enabling transmitted signals to better withstand the effects of various radiotelephone channel impairments, such as noise, fading, and jamming. Typically, channel coding reduces the probability of bit error, and/or reduces the required signal to noise ratio usually expressed as bit energy per noise density (Eb/N0), to recover the signal at the cost of expending more bandwidth than would otherwise be necessary to transmit the data signal. For example, Walsh codes can be used to channel code a data signal prior to modulation of the data signal for subsequent transmission. Similarly psuedo-noise (PN) spreading codes can be used to channel code a data signal.
A typical CDMA transmission involves expanding the bandwidth of an information signal, transmitting the expanded signal and recovering the desired information signal by remapping the received spread spectrum into the original information signals bandwidth. This series of bandwidth trades used in CDMA transmission allows the CDMA communication system to deliver a relatively error-free information signal in a noisy signal environment or communication channel. The quality of recovery of the transmitted information signal from the communication channel is measured by the error rate (i.e., the number of errors in the recovery of the transmitted signal over a particular time span or received bit span) for some Eb/N0. As the error rate increases the quality of the signal received by the receiving party decreases. As a result, communication systems typically are designed to limit the error rate to an upper bound or maximum so that the degradation in the quality of the received signal is limited.
In current CDMA communication systems, such as those defined by IS-95A ("Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System and published by the Electronic Industries Association (EIA), 2001 Eye Street, N.W., Washington, D.C. 20006) for Digital Cellular Systems (DCS) and ANSI-J-STD-008 for Personal Communication Systems (PCS), the base station and mobile station can connect to each other through multiple radio resources. The method for determining if new CDMA channels are to be added or released is called hand-off detection. Hand-off detection is typically performed on the traffic channel by the mobile station. The mobile station measures the forward (base station to mobile station) pilot channel, and indicates to the base station when conditions permit the addition or deletion of a forward and reverse link to a base station. The base station then allocates resources and assigns the mobile station one or more forward traffic channels via a Hand-off Direction message. In other possible implementations, the base station may perform the handoff detection by instructing the mobile station to transmit at a fixed power level, and simultaneously measures the received power from the mobile station. The base station then measures the signal transmitted from the mobile station to determine if a handoff is required.
Unfortunately, in current CDMA communication systems, there is no efficient way for the mobile station to change frequencies to scan pilot channels nor transmit at fixed power levels on alternate frequencies due to the continuous transmission of the forward traffic channel. Typically, this problem can be avoided by placing identical pilot channels on the same frequency and using the mobile station's same-frequency scanning capability to trigger the hand-off to the other frequency. This method is impractical, however, because it requires the base station to transmit pilot channels on all alternate cells in which the particular frequency covers in order to trigger hand-offs to other frequencies. Another solution is to allow the mobile station to vacate the traffic channel briefly to employ handoff detection techniques on other frequencies. This method, however, introduces gaps in voice coded speech which are discernible by the end-user.
Thus, a need exists for an apparatus and method which overcomes the deficiencies of the prior art.